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Ecology and Epidemiology

Apple Powdery Mildew Disease Progress on Sections of Shoot Growth: An Analysis of Leaf Maturation and Fungicide Effects. N. Lalancette, Jr., Graduate research assistant, Department of Plant Pathology, Pennsylvania State University, Fruit Research Laboratory, Biglerville 17307; K. D. Hickey, professor, Department of Plant Pathology, Pennsylvania State University, Fruit Research Laboratory, Biglerville 17307. Phytopathology 75:130-134. Accepted for publication 23 July 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-130.

The separate and combined effects of leaf aging and fungicide application on the incidence of apple powdery mildew were studied by monitoring disease progress on sections of shoots. In 1980, 1981, and 1982 six cultivar Rome Beauty trees were sprayed to run off at 2-wk intervals with the fungicide bitertanol at 150 mg ai/L; six unsprayed trees were used as controls. Vegetative terminals equally spaced around the periphery of each tree were chosen for observation. The youngest, most-unfolded leaf was tagged during each of the first three of five disease assessments made in each year; thus, by the end of the season three sections of shoot were demarcated. Analysis of disease incidence on each section revealed that disease severity first increased then decreased. Although bitertanol significantly reduced the rate of disease increase during the early phase of the epidemic, further reduction in disease incidence during the later phase was attributed to leaf maturation. We postulate that an increase in resistance during leaf aging deleteriously affects established fungal colonies. Approximately 46 and 39% of the untreated leaves harboring colonies during the early stages of the epidemics in 1980 and 1981, respectively, were no longer visibly infected by the last assessment.